Safety on the Jamarat Bridge

During the Hajj, Muslims travel to the city of Mecca in Saudi Arabia—for them, it’s a holy pilgrimage. Millions go every year; it happens between December and January during the 11th month of the Islamic (lunar-based) calendar, which means it moves around on the Julian Gregorian (solar-based) calendar. One of the phases of the trip is a walk across the Jamarat Bridge, from which people throw rocks at pillars said to represent the devil. The bridge is too narrow—in 1997 and 1998 people were killed when the crowd surged. In 2004, 251 pilgrims were trampled to death, and in 2006, another 362 died.

This year: 3.8 million pilgrims showed up, and everyone’s okay. Why? In part because the Saudi government implemented some crowd control recommendations from Dirk Helbing and Anders Johansson at the Dresden University Institute of Technology. Helbing and Anders looked at surveillance imagery from the 2006 trampling incident as if every person was a particle in fluid-dynamical flow. The stampedes, they reasoned, happen when laminar flow—smooth—transitions to stop-and-go and turbulent.

You might have seen this kind of research before. In fact, Wired did a Start item on it a couple years ago. But this time Helbing’s team noticed something new: As the crowd got heavier, it slowed down on the Jamarat Bridge—stop and go. But then as even more people walked on, the crowd clumped into turbulent nodes that spread shock waves forward and back. Nobody had ever seen turbulence develop in human crowds before (obviously it’s tough to test this kind of thing in human beings).

The authors quote from a book on crowd mechanics to describe what happens next:

“At occupancies of about 7 persons per square meter the crowd becomes almost a fluid mass. Shock waves can be propagated through the mass, sufficient to…propel them distances of 3 meters or more….People may be literally lifted out of their shoes, and have clothing torn off. Intense crowd pressures, exacerbated by anxiety, make it difficult to breathe, which may finally cause compressive asphyxia. The heat and the thermal insulation of surrounding bodies cause some to be weakened and faint. Access to those who fall is impossible. Removal of those in distress can only be accomplished by lifting them up and passing them overhead to the exterior of the crowd.”

(This, parenthetically, is exactly why I used to hate going to Little Italy during the San Gennaro Festival. I was always sure the crowd was about two more people away from going supercritical.)

In any case, Helbing’s group found that if the number of people passing a given spot in a given amount of time falls too low, the transition to turbulence was about a half hour away. They suggested to the Saudis—who followed up—that they change the shape of the Jamarat Bridge and its approaches, institute automated counting to head off the turbulence transition, and make a few alterations in the schedule of the Hajj.

"Could I feel comfortable if people had died and I’d declined to help?" Helbing told Nature’s Philip Ball. "It was a matter of responsibility."

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